The proliferation of wireless devices in computer networks has created a significant problem in the synchronization and secure interconnection of devices. Most wireless devices today are digital, using radio waves to communicate. A typical professional utilizing wireless devices today has a pager which receives digital messages, a digital cellular phone and a notebook computer with a wireless modem to retrieve and send e-mail. To connect to the office or other networks requires special hardware (such as adapter cards having transmission mechanisms) designed to connect to a wide-area or local-area network, which will then allow wire line access to the resources that the professional worker is accustomed to accessing.
A standard has been proposed for the merger of mobile communications with mobile computing. This standard, referred to herein as ‘Bluetooth’, proposes the incorporation of a small, inexpensive radio into every mobile device. Since this radio is designed to a standard, the mobile device and radio combination can then be optimized to reduce interference. The optimization is feasible since there is a common wireless protocol implemented in a single radio frequency band, rather than the multitude of optional devices using diverse technologies in various radio frequency bands available for wireless access today. The small, low-powered radio is intended for distribution in a module or chip that will communicate with other ‘Bluetooth’ enabled products. The Bluetooth standard is defining the communications between two selected devices and/or multiple selected devices. Further information regarding the Bluetooth standard is available at their website at http://www.bluetooth.com.
The standard currently defines the use of an available, unlicensed 2.4 GHz radio band that can support both voice and data exchange. While numerous commonly agreed-upon radio frequencies would work, this particular portion of the radio spectrum appears to be available worldwide for low-power unlicensed use. With a 0-dBm transmitter, this low-powered radio will be effective to establish networks of devices within about a 10 meter radius, with rapid degradation as the distance increases. With a 20-dBm transmitter the effective radio range will be about 100 meters. The low-powered radio module is intended to be built into mobile computers, mobile phones, 3-in-1 phones, printers, fax machines, modems, network interfaces (such as LAN or WAN connections), digital cameras, pagers, headphones, etc. Speeds of up to 721 Kbps for asymmetrical asynchronous data transmission, or up to three isochronous 64 Kbps voice channels, or a combination of voice and data channels totaling less than 1 Mbps symbol rate per picocell, are currently supported by the specification, and it is expected that the communication speeds will increase as the technology advances. Because Bluetooth uses frequency-hopping, several uncoordinated picocells can coexist within radio proximity of each other.
While this specification describes a major leap in the ability of devices to interact, there is still a significant problem with the establishment of secure channels for the devices. The specification allows the hand held or wireless devices to connect into what we will term a “piconet” or “picocell”. The picocell is just a physically proximate (or small) network. This piconet replaces cables for interconnecting physically proximate devices (within the above-described radio range). An ‘access point’ (or wireless device) with a Bluetooth radio can attach a picocell to an enterprise LAN or WAN. Deploying these new devices in an enterprise uncovers several unique security and management issues.
Prior art in this area, such as the above specification, defines methods for authentication and encryption at the baseband (physical) layer of the device, but these methods have heretofore-unrecognized limitations, which will be analyzed below. All of the prior-art methods that will be described have the goal of securely providing a secret cryptographic key to both devices that is then used with suitable cryptographic means to perform authentication and encryption. These methods differ as to the manner in which the key is obtained. They also differ as to their policies regarding the reuse of keys or their precursor PIN codes.
A first typical method that the prior art allows for is for two devices to receive, through some unspecified external means, a secret key known only to them. This method might be appropriate for two devices that are manufactured to be permanently paired with each other. They can store this key in association with the partner device's identifier and reuse the key every time they wish to communicate. If no method is provided for changing the key, the two devices are permanently paired with one another and can never be paired with other devices that received a different permanent key at the time of manufacture. One drawback of such a policy of key reuse is that the security association between the two devices is permanent. Another drawback is that if a third party was somehow able to learn the key, it would be able to impersonate another device or eavesdrop on the two devices at will thereafter. In all these scenarios, the third party could even impersonate or eavesdrop unobserved, since radio frequency communications in the intended RF spectrum can penetrate sight-barriers such as buildings and walls.
A second method often described, slightly more secure than the first, might be appropriate for two devices that are to be exclusively paired with one another on a long-term basis, such as a personal computer and its wireless mouse, or a cellular telephone and its wireless telephone headset. This method requires both devices to be provided with the same string called a “PIN”. The PIN may be provided by the manufacturer, or entered at each device by a user. The prior art defines how the PIN is combined with certain known, fixed data and certain ephemeral data to generate a secret key that is subsequently used for authentication and encryption. The precise details of how that occurs are not important here. Both devices wishing to create a long-term “pairing” relationship store the key associated with the paired device. The PIN that was used to generate the key is no longer needed, and can either be kept or discarded. This stored key is then reused anytime the paired devices wish to communicate securely. If a device changes ownership, it is possible to delete the prior key, enter a PIN for a new pairing relationship, and create and store a new key. One drawback of this method is that if a third party somehow learns the PIN, such as by eavesdropping on a verbal exchange or keypad entry, it can learn the key by eavesdropping on the pairing flows. Once it knows the key, it can impersonate another device or eavesdrop on encrypted communications.
A third variation provided by the prior art might be appropriate for two devices that wish to trust each other only for the duration of a single transaction or data exchange. In this method, the user enters a PIN on both devices just prior to the transaction. The PIN is used, as above, to generate a key. The key is used for authentication and encryption for the transaction, but both the PIN and the key are deleted after the transaction. If the two devices wish to do another transaction sometime in the future, both must be configured with a PIN again, a process that is burdensome to the user.
In a less-secure variation of this third method, a device stores the PIN in association with an identifier for the partner device, but deletes the key after use. Thus it reuses the same PIN whenever communicating with the same partner, but generates a fresh key before each communications session. The third method improves upon the security of the second method by changing the key frequently, thus limiting the duration of time that a third party could violate security if it is successful in learning the PIN and eavesdropping during the pairing flows.
A fourth method known in the prior art is to request baseband authentication and encryption, but to generate a key for each new communications session using a zero-length PIN. This method might be chosen by a manufacturer who wants their product to work immediately upon removal from the shipping box, without any configuration by the user, and wants to provide a minimal level of security. The drawbacks of this approach are similar to those of the third method, in that any third party who knows that a zero-length PIN is in use could eavesdrop on the pairing flows and learn the secret key, enabling it to impersonate another device and/or eavesdrop on encrypted communications.
Clearly a method that obtains the key through a non-secure exchange has some potential for impersonation and eavesdropping. Current art suggests verbally telling another person the key or PIN number, or delivering it on a piece of paper or via e-mail, so that the secret may be entered on each device by that device's user. If this verbal, paper, or e-mail exchange is observed by a third party, the secret may be compromised. A slight improvement is to restrict knowledge of the key or PIN to a single person, who enters it on a keypad on both devices. This eliminates overhearing or seeing the key or PIN, but the keypad entry itself may be observed by a third party, such as by using a hidden camera. A method that generates a secret key for each communications session or transaction using a piece of data exchanged in an insecure manner is somewhat more secure, but still subject to impersonation and eavesdropping, should a malicious third party eavesdrop on the key generation and exchange process. In the event a third party somehow acquires the secret, clearly a policy of reusing the secret has a greater potential exposure than if the secret is never reused.
The above described prior-art security methods are inadequate, burdensome, and unusable for mobile computers in an enterprise environment. An example of such a scenario addressed by the present invention is shown in Figure C.
In FIG. 3 there exists a server 301 that is connected to a typical enterprise LAN 303. A second server 311 is connected to the first server 301 over a WAN and also connected, conventionally to a LAN 321. Wireless devices such as a wireless notebook computer 315 can connect with a wireless access point on the server 311. The wireless device can also send information over the air waves to a printer 313 directly (rather than transmitting the information to the server 311 and having the server use a conventional wire line connection to transmit the information to the printer 313).
Another scenario depicted in FIG. 3 includes a wireless notebook computer 309, a telephone 307, and a pager 305. In this scenario, all three devices could communicate such that the telephone 307 or pager 305 could send messages to the notebook computer C19 for logging on the disk of the notebook computer 309. A realistic example of this in the business world might be where someone is in a meeting and awaiting the arrival of some urgent e-mail. The system could be set-up such that when new e-mail arrived at the notebook computer 309 (either over a cellular modem or over a LAN attached to the notebook computer via a piconet), the subject or sender of the e-mail would be sent from the notebook computer 309 to the pager 305 over the piconet and the pager would vibrate and display the message. Alternatively, the computer could dial the wireless telephone and, using a text-to-speech function, read aloud from an urgent e-mail. Another useful scenario might be where a facsimile machine 317 had a wireless connection to a notebook computer 319 such that the user of the notebook could utilize the underlying telephone network attached to the fax machine to send information to others without having to plug and unplug cables from the mobile computer, or access a server which has a connection to the printer. The connection would be made wirelessly directly between the notebook computer 319 and the facsimile machine 317. Yet another useful scenario is where a cable modem or ADSL adapter in the home is provided with a wireless transceiver, such that all type of devices in the home—such as personal computers, telephone handsets, television receivers, video recorders, audio speakers and audio recorders—can access the wire line network by means of a wireless connection. This offers a great convenience to users in that devices can easily be added or moved without the inconvenience and expense of cables or in-premises wiring. It is also desirable from the manufacturer or service providers point of view, since it allows for the consolidation of multiple services in a single physical access device.
The problem that the prior art fails to address becomes extremely apparent when one considers an enterprise scenario. “Enterprise” as used here refers to a very large-scale computer installation or network, such as is typically deployed by very large companies or organizations with thousands to hundreds of thousands of employees. Due to their sheer size or because they are active in several geographical locations, enterprises often have numerous smaller sites and/or large campuses housing thousands of employees. Such sites and campuses are generally interconnected by networking facilities such that an employee traveling from one site to another can gain access to application programs, resources, databases, and other computer facilities needed to do their job at any company location. In an enterprise scenario thousands to hundreds-of-thousands of users will roam among several to thousands of sites carrying wireless devices, each wishing to connect wirelessly in an unplanned ad-hoc manner to several devices throughout a given day. “Roam” as used here refers to a user physically moving himself and his mobile device containing a radio module from one location to another.
Because of the personal computer's multi functional character (i.e. a PC usually runs many different programs that exchange data with many different applications and devices on behalf of many different users), a personal computer user's security needs run the gamut from completely untrusted to totally trusted, which further complicates matters. The previously described state-of-the-art technology provides several ways to implement security policies, but none is satisfactory for this enterprise context. Let us examine whether any of the previously-described art can be used by a network administrator to limit access to a network.
1. Devices could be permanently paired with one another by the manufacturer, but this is inflexible and prevents a device from having multiple communication partners.
2. Devices could have long-term pairing relationships with specific other devices, for example by entering a common PIN at both devices, from which a key could be created for storage and reuse, or a fresh key generated for each communication session. Besides the drawbacks previously listed, this policy does not meet the needs of a PC to have different levels of security for different communication partners and, indeed, for different transactions with the same partner.
3. The administrator could configure all network access points with the same PIN, then provide the PIN to all possible mobile computer users that are allowed access. This minimizes the administrator's configuration effort since there is only one PIN to set up (albeit at multiple access points), and allows a properly-configured PC to roam anywhere in the enterprise and gain access through any access point, but if the secret PIN is compromised, the malicious third party could gain access to all access points. If an authorized employee quits the company, there is no easy way to revoke his access. This scheme is unacceptable because it is so insecure.
4. The administrator could configure each network access point or group of access points with a different PIN, then provide the PINs of certain access points to certain sets of authorized users. If an unauthorized person learns a PIN, he gains access to a set of access points. Managing lists of PINs at numerous mobile computers becomes difficult. Revoking a user's access privileges is difficult if the user retains the access device. The administrator could change the access points' PIN to bar an unauthorized user, but this forces all authorized users to simultaneously update their configurations. If the administrator wants to add a new network access point with a new PIN, all authorized users must be notified and must update their PCS. Giving a user access to different groups of access points, e.g. during travel, is difficult. Clearly this scheme is unworkable.
5. The administrator could assign a unique PIN to each mobile PC, and configure lists of authorized PINs at specific access points. Management is even more difficult. If the lists include all users, they may become unmanageably long, and also add to the cost of the access point devices since additional memory must be provided to store a large number of PINs. If the lists contain subsets of users, then a user's ability to roam is limited. If a user is added or removed, the administrator has to update information at all relevant access points. This method is relatively secure, except that if a person gains knowledge of the access lists configured at any access point, he could gain access to multiple access points by impersonating another device or misappropriating another user's PIN.
As is apparent from the foregoing, short-range wireless mobility presents a significant security challenge to enterprise network administrators. This is addressed by the present invention.